Spatial and temporal variations in d13C of dark respired CO2 (d13Cres) and total and water-soluble organic matter (WSOM) were analysed in four plant species. There was an increase in d13Cres over the light period (measured 5 min after darkening) in leaves, whereas no distinct diurnal pattern was detected in roots. Furthermore, large differences in d13Cres were found along the plant axis during day time. The amount of daily d13Cres enrichment in leaves relative to the putative substrate (WSOM) varied largely between species (3.2–15.9‰), probably due to different carbon allocation strategies. Positional pyruvate labelling was used to detect whether variations in d13Cres were related to changes in the relative activity of pyruvate dehydrogenase (PDH) and Krebs cycle (KC). The results indicate that one reason for the increase in d13Cres in leaves during daylight is an increasing carbon flux through the PDH relative to the KC. Labelling experiments revealed no clear diurnal variations in PDH and KC activity in roots. Further, we found new evidence that the fractionation process that leads to the diurnal d13C increase in leaf dark respiration is related to the well known 13C depletion of leaf material compared with heterotrophic tissues.
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Wegener et al. (2010) studied this question.